The term ‘superheated water’ (alternatively subcritical water) refers to liquid water under pressure between 100oC and its critical temperature, 374oC. The minimum pressure required to maintain the liquid state for a particular temperature is the vapour pressure at this temperature and can be calculated with the Extended Antoine Equation below1.
1Poling, B.E., Prausnitz, J.M., O’Connell, J.P. The Properties of Gases and Liquids, Fifth edition, McGraw Hill, (2001) p7.7
At lower temperatures and for most of this temperature range, the pressure of the medium does not have much effect on its properties, provided it is high enough to maintain the water in the liquid phase. Up near the critical temperature, the medium becomes very compressible and it has some of the properties of a supercritical fluid, and so the pressure does become important.
Calculation of Enthalpy
Methods for calculating the properties of water are continually reviewed by the International Association for the Properties of Water and Steam, who provide the equations for calculating steam tables (http://www.iapws.org). For approximate calculations over the range 10 to 300°C the calculation below can be used for the enthalpy of water. This is enough to provide an indication of the heat requirements; individual enthalpy values are within 1.5% of the steam table values. Enter the start temperature, the final temperature (Temperature 2) and the calculator will display the enthalpies at the two temperatures and the difference betweeen them.
|Enthalpy 2||0.0 kJ/kg|
|Enthalpy Difference||0.0 kJ/kg|